The present invention relates to a method of preparing pure sodium polysulfides directly from higer sodium polysulfides and sodium.
DE-PS 34 36 698 teaches a method of preparing sodium polysulfides, especially sodium tetrasulfide Na.sub.2 S.sub.4 and a polysulfide of the stoichiometric composition Na.sub.2 S.sub.3 from elements sodium and sulfur. It is said that the products are in very pure form and absolutely free of water and solvent. The prior art method is carried out by dosing sodium and the sulfur in a stoichiometric ratio corresponding to the desired polysulfide under a protective gas in alternating fashion into a prepared melt of a polysulfide. Vigorous agitation is used during the dosing operation. The portions are measured in such a manner that when the sodium is added, the reaction mixture remains in the state of an agitatable suspension, even if lower polysulfide intermediate products are formed which are present in the polysulfide melt in heterogenous phase. When the sulfur is added, the latter is allowed to react completely to a polysulfide of a higher sulfur content.
The particular polysulfide that is formed in this method thus depends on the particular setting of the stoichiometric ratio in the alternating charging of the reactants. The extremely vigorous reaction of the pure elements with one another is somewhat reduced, for example, in a preparation of sodium tetrasulfide by not bringing the sodium into direct contact with the elementary sulfur. Instead, the sodium is added in small partial amounts and in a thin jet stream into a vigorously agitated tetrasulfide melt heated to approximately 300.degree. C. (melting point Na.sub.2 S.sub.4 =285.degree. C.). As a result, intermediate, lower polysulfides, e.g. of the stoichiometric composiiton or empirical formula Na.sub.2 S.sub.3 or Na.sub.2 S.sub.2 and other polysulfides with higher melting points, are formed. These are reconverted back into the desired polysulfide, that is, Na.sub.2 S.sub.4 by post-charging of the required partial amounts of sulfur.
However, the alternating charging of the elements sodium and sulfur into the polysulfide turned out to be relatively time-consuming and labor-consuming in this known method. German patent application P 38 03 243, now U.S. Ser. No. 07/303,029 filed Jan. 30, 1989, assigned to the same assignee as the present application, describes a solution of this problem. The improved method provides that the sodium and the sulfur are delivered simultaneously in molten form to spatially separate locations of a cylindrical reactor whose diameter is dimensioned so that a reliable separation of the reaction zone between sodium and polysulfide from the reaction zone between sulfur and polysulfide is assured. The sulfur is added outside the reaction zone between sodium and polysulfide and in the agitated current as far as possible in front of the addition point for the sodium.
Both older methods share the common feature that sodium and sulfur are added to a melt, functioning as reaction medium, of a polysulfide obtained in a small amount by the direct reaction of sulfur with sodium in the desired stoichiometric ratio with alternating or simultaneous dosing.
Since a need was felt or perceived in practice to process higher polysulfides which had already been produced and stored or which stemmed from spent sodium/sulfur batteries into polysulfides with a defined, lower sulfur content, the industry has sought a simplified method for this purpose.
Another need arose regarding a solution for the handling of sulfur, which i critical on account of the high work temperatures in the preparation of disodium disulfide Na.sub.2 S.sub.2 (&gt;450.degree. C. ).